The present invention relates to an escapement arranged between a motor gear train and a roller to which there is attached the balance of a timekeeper, the balance being able to describe an arc of free oscillation and to receive oscillation maintaining impulses, this escapement including first and second toothed wheels meshing with each other, one of these wheels being driven by the gear train.
The Applicant of the present invention has already proposed an escapement that partly answers the definition that has just been given and a description of which is given in European Patent document No. 01200043.6, incorporated herein by reference. This document describes an escapement shown in FIG. 1, arranged, as is usual, between a gear train and a roller 2 supporting a sprung balance of a timekeeper. The sprung balance, not shown in the Figure, is able, as is known, to move through an arc of free oscillation and is arranged to receive impulses for maintaining these oscillations. By definition, the gear train, also called the train, is the set of wheels and pinions which, from a barrel, transmits the driving force to an escapement wheel. In FIG. 1, the gear train is represented by its last wheel 30, associated with pinion 31. Wheel 30 drives a first escapement wheel 3 via pinion 32 which is fixed thereto. First wheel 3 meshes with a second wheel 4. Wheels 3 and 4 have the same diameter and the same number of teeth.
The escapement also includes a rocking member 5 able to receive impulses generated alternatively by the first and second escapement wheels 3 and 4. Rocking member 5, in turn, transmits the impulses received to roller 2 to drive it in rotation so as to maintain the oscillations of the sprung balance fixed to said roller 2. Moreover, the rocking member is arranged to block alternately the first and second escapement wheels 3 and 4, after each impulse has been transmitted to roller 2.
Rocking member 5 fitted to the escapement is supported by an arbour 6 freely pivoting in a plate (not shown) comprised in the timekeeper. This rocking member 5 has a substantially triangular shape. A first apex of the rocking member defines an edge 19 on which a tooth 8 of first wheel 3 can rest to subject rocking member 5 to an impulse in a first direction. A second apex of the rocking member defines another edge 19 on which a tooth 10 of the second wheel 4 can rest to subject rocking member 5 to an impulse in a second direction B, opposite to the first direction. Edges 19 of the rocking member are connected by a rim 14 allowing first 3 and second 4 wheels to be blocked alternately, in proximity to the edges, after each transmitted impulse. Finally, a third apex of rocking member 5 comprises two teeth 11 and 12 capable of meshing with a tooth 13 of roller 2.
Each of teeth 8, 10 of wheels 3 and 4 has a front flank 16 comprising a first curved summit cut out portion 17 capable of abutting against rim 14 of the rocking member to block one of the wheels, and a second cut out portion 18 in the shape of an arc of circle, following the first cut out portion, capable of abutting against one of edges 19 to drive rocking member 5 in rotation.
It is an object of the present invention, which takes up the features disclosed in the aforecited document, namely two wheels meshing with one another, one of the wheels being driven by the gear train, to provide a timekeeper escapement having improved chronometric performances.
For this purpose, the escapement of the present invention thus further includes a rocking member having:
first and second contact means able to receive impulses generated alternately by the first and second wheels,
impulse means capable of co-operating with the roller to transmit said impulses to said roller to drive it in rotation and to maintain the oscillations of the balance,
and first and second blocking means for blocking alternately said first and second wheels after each impulse transmitted by said impulse means to said roller,
said first and second contact means and said first and second blocking means being arranged at the periphery of the rocking member in regions remote from each other.
According to a preferred embodiment of the invention, this rocking member is supported by an arbour pivoting freely in a plate of the timekeeper and the impulse means comprise first and second teeth capable of meshing with a tooth of the roller. It advantageously has a substantially symmetrical shape with respect to an axis passing between said first and second teeth of the rocking member and through the centre of the arbour, wherein:
the first and second contact means are arranged symmetrically with respect to said axis,
and the first and second blocking means are arranged symmetrically with respect to said axis.
Thus, the rocking member of the invention not only allows the angle of lift to be reduced, which allows the chronometric performance to be improved, but also the impulse speed to be increased by selecting the leverage. Indeed, the introduction, in the present invention, of a rocking member comprising contact means (and associated impulse) and blocking means separated from and set apart from each other and co-operating with a tooth of the other wheel allows:
the number of teeth of the escapement wheels to be increased,
the distance between the centre of the roller and the rocking member to be varied, and
the angle of attack of one tooth of the first wheel or respectively the second wheel on respectively the first and second contact means to be optimised, at the time of an impulse.
As a result, on the one hand, the peripheral speed of the rocking member impulse means can be selected so as to be greater than that of the contact means and, on the other hand, the force transmitted by the teeth of the first and second escapement wheels on the rocking member is substantially perpendicular to the radius passing through the rotational axis of the rocking member and the point of contact of the teeth on the rocking member, which allows maximum transmission of the drive torque.